I currently use combinations of FireBug and TextPad but would hate to miss out on the party if there are better options out there.
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I use NotePad++ and am happy (of course, that is when I am not using Visual Studio).
NotePad++ contains support for intellisense type feature as well.
(This is a cross-answer post)
I've tried out all of the above and my vote goes for Netbeans, which has been mentioned. However the answer didn't really sell you on the features which you can find here.
- Intellisense including jQuery built in
- Extended (Eclipse-style) documentation for functions
- Function and field outlining
- Code folding